The development of the scanning tunneling microscope has given rise to a family of scanned-probe microscopes that enable the visualization of submicron substrate features. Additional scanned probe systems have been developed to enable the imaging of surface-related magnetic forces and electrostatic forces. The electrostatic-force microscope employs a vibrating probe which bears an electric charge. The probe's vibration amplitude is affected by electrostatic forces from charges in the sample being scanned. The electrostatic force microscope thus enables the mapping of electrical properties of an underlying microcircuit on a very fine scale. Such microscopes have been used to measure charges, dielectric constants, film thicknesses of insulating layers, photovoltages, and surface electrical potentials.
Nonnenmacher et al. in "Kelvin Probe Force Microscopy", Applied Physics Letters Vol. 58, No. 25, 24 Jun., 1991 pages 2921-2933 describe the application of a capacitive force micrgscope to the determination of potential differences between a probe head and a surface being scanned. The microscope there described employed a modified Kelvin method wherein two conductors are arranged as a closely spaced parallel plate capacitor (one conductor being the probe). A periodic signal applied to the probe induces a force that causes the probe to vibrate at the applied frequency. As the force value is related to the difference in potential between the probe and the surface, the resulting vibration is indicative thereof. The actual measurement is created by adding an additional bucking voltage between the plates until the space therebetween is field free and the resulting force goes to zero. The bucking voltage is thus equal to the potential difference.
A similar system has been applied to the mapping of lateral dopant profiles in semiconductors (e.g. see "Lateral Dopant Profiling in Semiconductors by Force Microscopy Using Capacitive Detection" by Abraham et al., J. Vac. Soc. Technology B, Microelectronics, Process. Phenom. Vol. 9, No. 2, PT.2, Mar., April, 1991, pp 703-706). The system used by Abraham et al. consisted of a tungsten wire etched-cantilever that was several hundred microns in length and mechanically bent in the final 50 microns at a right angle. The tip served as a surface probe during imaging. Piezoelectric transducers scanned the sensor head above the sample. The tip was vibrated at one of its resonance frequencies and its resultant motion was measured using optical heterodyne detection. Shifts in the resonant frequency due to force gradients between the tip and sample caused changes in amplitude of the oscillations, which changes were used in a feedback loop to control the height of the tip above the sample. Surface images, thus obtained, showed contours of constant force gradient.
To measure dopant densities, the tip was biased with a DC voltage (chosen to deplete the surface of the semiconducting sample), plus an AC voltage to induce mechanical oscillation of the tip at a resonant frequency of the cantilever. Amplitude variations in the signal then corresponded primarily to variations in the depletion capacitance and therefore dopant concentration.
To achieve a separation of the measured values, Abraham et al. employed detection of two separate resonances of the cantilever (e.g. a fundamental and second harmonic). The movements of the cantilever in response to the second harmonic energization were small vibrations at the end of the cantilever and more substantial vibrations in its middle. The fundamental resonant vibrations caused substantial movements at the ends of the cantilever and smaller movements in the middle. Hence, for a given excitation, the second resonance was less sensitive than the first resonance.
The patented prior art relating to AFM systems is extensive. U.S. Pat. No. 4,668,865 to Gimzewski et al. describes a scanning tunnelling microscope wherein variations of tunnel current occurring between a sharp tip and a substrate are employed to image the substrate surface. U.S. Pat. Nos. 4,724,318 and Re. 33,387, both to Binnig, include one of the first descriptions of a scanning tunnelling microscope that employs tunnel current for image visualization. U.S. Pat. No. 4,851,671 to Pohl describes an AFM capacitive force measurement system wherein a probe is oscillated and the deviations of the frequency of oscillation are used both to control the distance of the probe from the surface being investigated and to plot an image of the contour of surface potential.
U.S. Pat. No. 4,941,753 to Wickramasinghe describes another AFM capacitive microscope of the absorption type. U.S. Pat. No. 4,992,659 to Abraham et al describes the application of an AFM to the measurement of magnetic fields above the surface of a sample and U.S. Pat. No. 5,047,633 to Finlan et al describes a multiprobe AFM for visualizing the topography of a sample being scanned.
A number of patents describe various methods of fabricating microtips and/or cantilevers for AFM systems, environmental sensing, etc. Such disclosures can be found in the following U.S. Pat. Nos.: 5,020,376 to Wall et al; 5,021,364 to Akamine et al; 4,670,092 to Motamedi; 4,968,585 to Albrecht et al; 4,943,719 to Akamine et al; 4,916,002 to Carver; 4,906,840 to Zdeblick et al. and 4,800,274. Similar types of structures for accellerometers are found in U.S. Pat. Nos. 4,783,237; 4,600,934; and 4,851,080.
Additional patents relating to STM and AFM instruments teach the coupling of an objective lens to AFM and STM systems, a micromechanical sensor head which measures AFM cantilever motion by the sensing of tunnelling current, a holding mechanism for probe tips in AFMs and STMs, and a fine adjustment mechanism for STMs. Those teachings can be found in the following U.S. Pat. Nos.: 5,041,783 to Ohta et al; 4,806,755 to Duerig et al; 4,883,959 to Oski et al. and 4,945,235 to Nishioka et al. STM-like probes also have been used to produce fine line patterns on insulating surfaces (see Hodgson et al in U.S. Pat. No. 5,047,649) and the concept of applying an oscillatory force to a pick-up (in other fields) is shown in Uchida, U.S. Pat. No. 4,424,583.
Accordingly, it is an object of this invention to provide an improved AFM system that is capable of making highly accurate topographical and electrical surface measurements of a sample.
It is another object of this invention to provide an improved cantilever beam probe for an AFM which enables decoupling of surface potential measurements from topographical measurements.
It is yet another object of this invention to provide an AFM which enables, through selective application of energizing frequencies, a sensing of changes of capacitance with respect to surface potential of a sample.